Theological Demon Concept
Throughout the centuries, these supernatural demons have plagued mankind with their wickedness. Countless stories can be found in cultures from all over the world. Although no two stories are ever alike, there's one theme they all have in common: The Oni are the evil of humankind.
As there are considered to be many different types of Oni, one that poses the greatest danger to our existence is one that is Earth-bound
And so it is written... When a human is so utterly wicked that their soul is beyond any redemption, It's transformed into an Oni during life, and remains on earth among the unfortunate living.